X-disease, or cherry buckskin
X-disease is caused by a phytoplasma microorganism that infects the phloem cells of cherry trees. Phytoplasmas are living organisms smaller than a bacterium and unlike bacteria have a unit membrane but no cell wall.
The X-disease phytoplasma is a major cause of decline and death of cherry trees and loss of fruit in the North Coast, Northern San Joaquin Valley, and Sierra Nevada foothills of California. The phytoplasma can also infect and damage peach trees growing near cherry. Apricot and prune can be infected but do not develop X-disease symptoms.
Identification
X-disease causes cherry fruit to be more pale than normal with stems that are short, stiff, and thickened. Affected fruit may have a leathery (tough) skin with a pebbly (rough) surface. Cherries fail to develop mature color and a normal sweet flavor. Unlike the spherical shape of healthy fruit, cherries affected by X-disease may be somewhat conical or pointed on the end opposite the stem.
Trees with X-disease have yellowish leaves in midsummer. Only one or a few limbs or the entire tree may be affected. On Mahaleb cherry rootstocks, fruit generally show no symptoms, but infected trees suddenly wilt and collapse and die above the graft union.
Collecting symptomatic fruit and sending it to a laboratory that can test for plant pathogens can confirm whether the cause of unhealthy fruit is the X-disease phytoplasma. On Mahaleb cherry rootstock removing a section of bark over the graft union and examining the wood is also diagnostic for X-disease. If the tree is infected with X-disease, where the rootstock and scion meet the wood will have a dark, jagged line that has the appearance of a zipper, a symptom called zippering.
Life cycle
The phytoplasma colonizes phloem cells of infected trees, damaging the plant’s vascular system. The pathogen is spread by certain leafhoppers, which acquire the phytoplasma when feeding on infected cherry trees or other plants that host the pathogen. The pathogen can also spread by grafting if a source plant is infected.
Damage
X-disease reduces the production of healthy cherry fruit and causes affected fruit to become unpalatable. The causal phytoplasma commonly kills infected cherry trees.
Solutions
There is no cure or treatment to remedy X-disease. Remove infected limbs or entire trees. Where there are multiple cherry trees with some unaffected, promptly removing the affected tree(s) reduces the likelihood that any healthy cherry trees nearby will become infected and diseased.
Avoid planting cherries near common alternative hosts that are fed upon by the same species of leafhoppers that feed on cherries, or remove the alternative hosts. These include bitter cherry, boxwood, ceanothus, chokecherry, crabapple, hawthorn, lilac, myrtle, privet, pyracantha, and viburnum.
Control weeds that can host both the X-disease phytoplasma and cherry leafhopper (Fieberiella florii). These include dandelion (Taraxacum officinale) and all clovers including burclover (Medicago polymorpha), clover (Trifolium species), and sweet clovers (Melilotus species). Control curly dock (Rumex crispus) because it is a host of the pathogen and mountain leafhopper (Colladonus montanus) that also feeds on cherry. In the Sierra foothills the Scaphytopius acutus leafhopper appears to be the most common vector of the X-disease phytoplasma. Hosts of S. acutus include red clover (Trifolium pratense) and various rosaceous species including apple, blackberry, peach, and strawberry.
Effective management of the disease through control of leafhoppers is not feasible for home fruit growers. Employ the cultural controls above and replace any trees that are not performing as desired.
Adapted from Integrated Pest Management for Stone Fruits, Pest Management Guidelines: Cherry, and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).
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Cherries with X-disease (right) compared with healthy fruit of the same maturity.
Healthy fruit clusters (left) and two clusters each with only two, pale, small fruit due to X-disease.
Adult (right) and nymph of cherry leafhopper, a common vector of the X-disease phytoplasma.
Adult (left) and nymph of mountain leafhopper, a common vector of the X-disease phytoplasma.
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